Abstract: In order to improve the thermal efficiency of radiant tubes, a flat double-P type radiant tube was designed, and five major axis-to-minor axis ratios of 1.0, 1.1, 1.2, 1.3 and 1.4 on the cross section of the center pipe were taken for numerical simulation to investigate the influence characteristics of the degree of flattening on the performance of the radiant tube by using the commercial software FLUENT. It is found that in the case of keeping the heat transfer surface area of the radiant tube invariable, when the degree of flattening increases, the radiant angle factor for the radiant tube and strip steel gradually grows, the radiant heat flow improves, and then thermal efficiency of the radiant tube rises. However, with an increase in the degree of flattening, the surface temperature difference of the radiant tube gradually increases, and the surface temperature uniformity significantly deteriorates after the degree of flattening reaches 1.3. In consideration of the thermal efficiency and surface temperature uniformity of the radiant tube, the better degree of flattening is 1.2. At this degree of flattening the radiation heat efficiency of the radiant tube increases by about 1% compared with that at the degree of flattening of 1.0, while the surface temperature uniformity is almost the same.